(1946) found ambiguous and variable re- 

 sults when examining the relation between 

 particle size and retention on the gill in 

 oysters. These results suggest that the 

 filtering efficiency of oysters is not 

 necessarily related to their pumping rate. 

 The role of mucous in actually trapping 

 food particles in oysters is unclear, as 

 is the importance of dissolved organic 

 material to the overall energy intake. 



The assimilation of significant lev- 

 els of dissolved organic matter (DOM) in 

 oysters was documented by Collier et al. 

 (1953), although the methods were criti- 

 cized by Galtsoff (1964). Oysters probably 

 "leak" some organic carbon (Johannes et 

 al. 1969). Some workers feel that hetero- 

 trophic microorganisms (bacteria) repre- 

 sent the only significant consumers (and 

 packagers) of DOM (Sottile 1973). 



Feeding activity in oysters is high- 

 est at low concentrations of food; there 

 is a negative correlation between pumping 

 rate and turbidity (Loosanoff 1962). The 

 effect of turbidity on the pumping rate is 

 illustrated in Figure 10 (Loosanoff and 

 Tommers 1948). Some ambiguity between 

 laboratory and field studies exists how- 

 ever; for example, oysters held above the 

 bottom, in the so-called maximum turbidity 

 zone, grew more rapidly than those on the 

 bottom in commercial beds in Buzzards Bay, 

 Massachusetts (Rhoads 1973). Reef oysters 

 may have a similar advantage in the study 

 area. The average suspended load of par- 

 ticulate organic matter (POM) in a typical 

 estuary in Georgia ranges between 4.6 and 

 15.8 mg/liter afdw (Odum and de la Cruz 

 1967). Hanson and Snyder (1979) reported 

 extraordinarily high levels of suspended 

 particulate organic carbon (POC) in the 

 study area (0.02 to 0.1 gC/liter), equiva- 

 lent to approximately 40 to 200 mg POM and 

 much higher than the 1967 estimate of Odum 

 and de la Cruz. High levels of suspended 

 organic matter could reflect strong tidal 

 currents. 



Particulate organic matter is a mix- 

 ture of marsh plant detritus, phytoplank- 

 ton, benthic algae, bacteria, zooplankton 

 (incuding oyster larvae), and DOM adsorbed 

 onto clay particles. An intertidal oyster 

 diet is a mixture of these items, some of 

 which are not incorporated into oyster 

 tissue while others are more assimilable. 



The presence of cellulolytic activity in 

 the crystalline style of the oyster has 

 been reported (Newell 1953), but the 

 amount and kind of cellulose that can be 

 used by the animal are unknown. Because 

 the diet of the oyster includes dinofla- 

 gellates and other algae with cellulose 

 tests (outer covering), the ability to 

 digest such structural polysaccharides 

 appears to be advantageous. 



Results from laboratory experiments 

 on oyster feeding are sometimes ambiguous 

 or at least not directly applicable to 

 oysters in their natural setting. For 

 example, a study by Epifanio (1979) indi- 

 cated that the gross chemical composition 

 of experimental algal cultures fed to oys- 

 ters (protein, lipid, carbohydrates, and 

 ash) was less important to subsequent oys- 

 ter growth than was the specific type of 

 algae used. Oysters have even been shown 

 to grow on cornstarch-supplenented diets 

 (Ingle 1967). 



A final note on the specific diet of 

 intertidal oysters: in the only analyses 

 of 6^^C (stable carbon isotope ratio test) 

 of oyster tissue from the Duplin River, 

 Georgia, Haines (1976) and Haines and 

 Montague (1979) found the stable carbon 

 isotopic ratio ranged from -21°/oo to 

 -24°/oo, typical of organic matter pro- 

 duced by phytoplankton. The interpreta- 

 tion indicates that oysters, even in small 

 tidal creeks surrounded by Spartina , feed 

 only on algae. We think this interpreta- 

 tion should be accepted cautiously due to 

 discrepancies found in different tissues 

 of shrimp. (Brian Frye, University of 

 Texas Marine Science Institute, Port 

 Aransas; pers. comm. ) 



The rate at which intertidal oysters 

 ingest particulate matter is the product 

 of four factors: (1) the average rate 

 (volume/time) at which they can clear the 

 water of POM of a favorable size range; 

 (2) the concentration of suspended food in 

 this size range; (3) the total time that a 

 given oyster (or reef) is inundated; and 

 (4) the percentage of inundation time that 

 oysters filter water. Any significant up- 

 take of DOM would add to this total rate. 

 An energy budget for individual oysters is 

 included in the Appendix and summarized in 

 Section 2.5; energy requirements of a unit 

 area of reef are discussed in Chapter 3. 



26 



